The present invention relates to a method for digital measuring a capacitive sensor. The invention relates also to an arrangement for digital measuring a capacitive sensor. The arrangement is provided with a charge balance frequency converter having an operational amplifier with an inverting input, a noninverting input and an output. Between the output and the inverting input an integrating capacitor is connected and the noninverting input is connected with a reference potential.
In more detail, the invention concerns a capacitance measurement of a capacitive sensor, which is applicable to the determination of the dielectric constant of a liquid, in order to determine the characteristics of the liquid, and concerns a procedure, which allows to compensate the disturbing conductivity during the measurement of the capacitance and to determine the conductivity parameters beyond that quantitatively. Typical application areas of the invention are the determination of the alcohol content in the fuel mixture of combustion engines or the determination of the quality and the level in storage vessels of liquids.
For the direct conversion of the capacitance of the capacitive sensor to a digital value the Switched—Capacitor (sc) technique offers very robust solutions. Well-known A to D conversion techniques, like the sigma delta or the charge balancing procedure have been modified by replacing one of the capacitors of the sc network with the sensor capacitor. When an unknown capacitor and a known reference capacitor charge and discharge an integrating capacitor respectively controlled by a feedback loop, so that the overall net-charge is balanced to be zero, the value of the unknown capacitor can be determined as a digital number by counting the number of discharge events This is disclosed in U.S. Pat. No. 5,990,578.
Compared to other common methods of measuring capacitive sensors like C-f-conversion with following f-V-conversion and AD-conversion or the AC-based measurement of impedance, a direct capacity to digital conversion simplifies the sensor readout circuit substantially.
A very fortunate design is possible if the sensor is a two terminal floating capacitor because the measurement can be made insensitive against parasitic capacitances to ground as disclosed in U.S. Pat. No. 5,990,578. However, it is also feasible to use a grounded sensor capacitor in a switched capacitor design.
Usually switched capacitor networks do not allow for resistive components because any continuous current causes errors. The network equations are based on the presumption that all node voltages come to a complete settling during one half of each clock cycle.
For that reason the measurement of capacitive sensors, which include shunt resistors, e.g. capacitive liquid level sensors, is usually based on an ac measurement of magnitude and phase or on resonance circuits as shown in DE 199 17 618 B4. Those circuits include the synthesis of sinusoidal waveforms, precise synchronous demodulation and a-to-d conversion or inductors in case of the resonance method.
As shown in U.S. Pat. No. 4,971,015, another approach, to select the measuring frequency high enough that the resistive portion can be neglected does not satisfy the demand for a wide capacitance and conductance range. In numerous publications a number of relaxation oscillators are used to determine the resistive portion and compensate its influence. This is disclosed in US 2004/0251919 A1 and in US 2004/0004487. An empirical approach is usually required in order to get satisfying results with that method.
In WO 2009/030743 a sc-based technology is described, where the current caused by the shunt resistance is compensated by means of a controlled current source. In that case a sc amplifier establishes an analog output voltage in a single clock cycle, which is digitized thereafter by an AD-converter. In order to guarantee the correct value for the compensation current, in a separate regulation loop the voltage change over the measuring capacity during the sample is used as control signal. Altogether this method is relative complex and likewise not very robust against disturbances. A substantial disadvantage is also that only a floating sensor capacitor can be used.